The art of lithographic printing is based on the immiscibility of ink and water. A lithographic printing plate is composed of ink receptive regions, commonly referred to as the “image area,” generated on a hydrophilic surface of a substrate. When the surface of the printing plate is moistened with water and printing ink is applied, revealed portions of the hydrophilic surface retain the water and repel the printing ink, and the oleophilic image area accepts the printing ink and repels the water. The printing ink retained on the oleophilic image area may then be transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the desired surface.
Lithographic printing plates typically comprise a radiation-sensitive coating applied over the hydrophilic surface of a substrate. Conventional radiation-sensitive coatings include photosensitive components dispersed within an organic polymeric binder. After a portion of the coating is exposed to radiation (commonly referred to as imagewise exposure), the exposed portion becomes either more soluble or less soluble in a developer than an unexposed portion of the coating. A printing plate is generally considered a positive-working plate if, after exposure to radiation, the exposed regions or areas of the radiation-sensitive coating become more soluble and are removed in the developing process to reveal the hydrophilic surface. Conversely, if the exposed regions or areas become less soluble in the developer and the unexposed regions or areas are removed in the developing process, the plate is considered a negative-working plate. In each instance, the undeveloped areas that remain on the plate provide an ink-receptive image, while the revealed regions of the substrate's hydrophilic surface repel ink.
High demands are placed on radiation-sensitive coatings used for printing plates. Heretofore, two avenues have been taken to improve the properties of these coatings. The first avenue concentrates on improving the properties of the photosensitive components of the coatings. The second avenue involves improving the properties of the polymeric binder that controls the physical and mechanical properties of the coating. The latter avenue has been the source of significant research and innovation because the behavior of radiation-sensitive coatings in the imaging, developing and printing processes, as well as the shelf life and durability of the printing plate are heavily influenced by the choice of binder material.
These polymeric binders include various substituents having differing effects on the physical and mechanical properties of the binder. For example, hydrophilic structural elements such as carboxyl groups, hydroxyl groups, etc., generally promote good developability of the photosensitive compositions in aqueous-alkaline developers and contribute to adequate adhesion to traditional substrates. On the other hand, hydrophobic structural elements hinder the developability in common developers, but ensure good acceptance of the ink in the printing process, which is absolutely essential for printing plates.
Given the broad spectrum of requirements for polymer binders, there has been much work on the synthesis and optimization of the use of these binders for photosensitive compositions, see for example, H. Baumann and H. J. Timpe, Chemical Aspects of Offset Printing, J prakt. Chem./Chemiker-Zeitung, Vol. 336, pgs. 377–89 (1994). Below is a summary of numerous types of previously reported polymer binders. As noted, each of these binder types suffer from some drawback resulting from the need to balance the broad spectrum of desired properties required for printing plates, including adherence to the substrate, durability, photosensitivity, developability, and ink-receptiveness.
U.S. Pat. Nos. 4,511,640, 4,618,562, and 4,731,316 describe binder systems that are composed of mixtures of various polymers, possessing different hydrophilic/hydrophobic properties. These mixtures have drawbacks in that separation of the mixture often occurs during the coating process. Furthermore, separation of the hydrophobic polymers in the development step may result in silting in the development machines and redeposition of dispersed elements of the coating on the surface of the photosensitive material.
Moreover, various copolymers containing monomer units having low hydrophilic properties, such as styrene, acrylate, methacrylate, etc., combined with more hydrophilic comonomers have been reported. Examples of such comonomers are semi-esters of maleic acid (Canadian Patent 1,172,492 and U.S. Pat. No. 4,687,727), itaconic acid (U.S. Pat. No. 5,260,161), and acrylic acid or methacrylic acid (EP 487 343A1 and U.S. Pat. Nos. 4,304,832 and 4,123,276). The drawback of such copolymers is the potentially narrow margin of applicability for properties such as film adhesion, developability, ink acceptance, and plate life.
U.S. Pat. No. 4,177,073 describes a photosensitive composition in which the binder is a reaction product of cellulose esters with cyclical, intramolecular acid anhydrides of dicarboxylic acids. However, these binders may not be sufficiently oleophilic for use in printing plate formulations. Furthermore, their printing plate life may not meet the demands of a modern printing plate.
Acetals of aliphatic aldehydes with unsubstituted lower alkyl group and vinyl alcohol/vinyl acetate copolymers have been reported (U.S. Pat. Nos. 2,179,051 and 4,665,124). But such binders may cause problems during the development of printing plates due to an insufficient proportion of hydrophilic groups in the polymer. As an improvement, partially acetalized vinyl alcohol/vinyl acetate copolymers have been provided with hydrophilic or alkaline-soluble groups by additional reactions. U.S. Pat. No. 4,940,646 reports aldehyde-containing hydroxyl groups that are used for the acetalization, besides aldehydes with hydrophobic groups (e.g., alkyl or aryl groups). Yet this structural change may not result in a distinct improvement in developability.
In order to improve developability, sulfonyl urethane groups were introduced into polyvinyl acetals as reported in U.S. Pat. Nos. 3,372,105, 3,732,106, and 4,387,151. However, the low acidity of these groups requires developers with a large amount of solvent. Furthermore, the microelements of the resulting printing plate may have poor adhesion and may be easily abraded in the printing process.
GB 1,396,355 and U.S. Pat. No. 3,847,614 describe binders that may be produced by acetalization of saponified copolymers of vinyl acetate and a carboxyl group-carrying monomer, such as crotonic acid. However, this type of binder may result in systems with poor photosensitivity and short printing plate life. Moreover, such compositions may be poorly developable in aqueous-alkaline developers, because the acid number of the binder is greatly decreased by chemical reactions of the carboxyl group during the saponification and/or acetalization process.
U.S. Pat. Nos. 5,045,429 and 4,681,245 describe compositions including carboxyl groups formed by reacting acetals, which have been separately produced from aliphatic aldehydes and polyvinyl alcohol, with intramolecular cyclical acid anhydrides of dicarboxylic acids. However, the synthesis may be costly because the reaction of the acid anhydrides is only possible in aprotic solvents. Furthermore, the photosensitivity of the compositions prepared from this type of binder may be too low.
U.S. Pat. No. 4,741,985 describes binders produced in a three-stage synthesis, starting with polyvinyl alcohol. The first stage is acetalization with aliphatic aldehydes. The second stage is reaction with intramolecular cyclical acid anhydrides of dicarboxylic acids. The third stage is partial esterification of the carboxyl groups with substituted alkyl halides. Despite the high expense in the synthesis of this binder material, the photosensitivity of the resulting film may be inadequate.
The expense of a multi-stage synthesis may be avoided if, as described in U.S. Pat. Nos. 5,219,699, 4,652,604, 4,895,788, 4,940,646 and 5,169,897, polyvinyl alcohol is reacted with aliphatic aldehydes and carboxyl-containing aliphatic or carboxyl-containing aromatic aldehydes. However, the photosensitive compositions prepared from this may not be suitably radiation-sensitive or developable.
Mixtures of polyvinyl acetals and organic polymers, which possess an acid number greater than 71 mg KOH/g, have also been described in U.S. Pat. No. 5,143,813. However, separation of the mixture from the substrate can occur during lamination or drying of the photosensitive compositions.
Thus, despite the significant amount of effort spent developing improved organic polymeric binders, reported binders tend to suffer from one or more drawbacks. As noted above, certain types of polymeric binders lack suitable adherence to substrates. Other polymeric binders lack suitable photosensitivity under conventional imaging conditions. Still other polymer binders have difficulty withstanding the extended exposure/development steps required in the production of certain printing plates, for example, flexographic printing plates. Further certain polymer binders have barely sufficient developer resistance after imaging, particularly IR imaging. Further yet, certain polymer binders have low resistance to the mechanical stress that plates are subjected to, as well as the chemicals used to clean and treat finished plates. Additionally, many of the polymer binders must be dissolved in organic solvents during manufacturing of the printing plate. Many organic solvents are being subjected to increased regulation due to environmental considerations.